Materials testing apparatus



March 31 1942. BERNHARDQ 2,277,813

MATERIAL TESTING APPARATUS Filed Dec. 18, 1957 2 Sheets-Sheet 2 ?atenieoi Mar. 31, N42

Y 2,277,813 MATERIALS 'rnsrmo APPARATUS Rudolf K. Bernhard, Ridley Park, Pa., assignor,

by mesne assignments, to The Baldwin Locomotive Works, a corporation of Pennsylvania Application December 18, 1937, Serial No. 180,491

(or. ass-14') 12 Claims.

Thi invention relates-generally to materials testing apparatus and more particularly toapparatus for subjecting a test specimen to a pulsating load between predetermined maximum and minimum loads. Various arrangements have been heretofore suggested and used for subjecting a specimen to pulsating loads, but such prior arrangements have been deficient in various respects including the inability to subject the specimen to a pulsating load along a true sine curve. Other deficiencies have been the inability to control the pressure limits with a high degree of accuracy and sensitivity regardless of whether th pressure limits were relatively low or high or of small or large differential.

It is anobject of my invention to provide an improved apparatus and control system, together with improved pressure responsive and pressure filtering means, whereby all of the foregoing deficiencies may be substantially if not wholly overcome while at the same time obtaining a. high degree of accuracy, sensitivity, stability and ease and rapidity of adjustment of the pressure limits as well as widely varying the pressure differential between such limits.

It is a. furtherobject of my invention to accomplish the foregoing desirable results in an apparatus which, considering the niceties of control and responsiveness necessary, is relatively economical in manufacture, operation and maintenance. In one speciflcaspect of myinvention I have provided a main pump for producing a predetermined minimum constant pressure and a pulsating pump for producing a pulsating pressure having a predetermined maximum pressure. These pumps are respectively controlled by mini mum and maximum pressure controllers.

Other objects and advantages will be more apparent to those skilled in the art from the following description of the accompanying drawings in which:

Fig. 1 is a diagrammatic outline of the elements embodied in my improved apparatus, parts of which are shown in section to disclose the mechanical details thereof;

Fig. 2 is an enlarged front elevation of one of the controllers, parts of which are broken away to show details of construction;

Fig. 3 is an enlarged sectional view of the main pump for establishing a constant pressure;

Fig. 4 is an enlarged sectional view of the pulsator pump Fig. 5 is a fragmentary diagrammatic view of a modified form of means for controlling the rate of flow from the main pumping unit to the hydraulic load producing apparatus;

Fig. 6 is another modification of means for controlling the rate of flow from the main pumping unit.

In the particular form of the invention, such as is disclosed herein merely for the purpose of illustrating one specific form among possible others that the invention might take in practice, I have shown a hydraulicloading apparatus such as a materials testing machine I in combination with a motor-driven main pump 2 preferably of the variable displacement Hele-Shaw type as shown in Patent No. 1,152,729, together with a motordriven variable displacement pulsator pump 3, a combined maximum pressure indicator and controller l, a combined minimum pressure indicator and controller 5, and a maximum and minimum pressure control or filtering valve mechanism 6. The testing machine disclosed herein is of the well-known Emery type disclosed in Emery Patent No. 1,848,468 having generally a. loading cylinder l and ram 8 and a hydraulic support ii. The rain 8 is formed as a fixed part of a loading frame comprising a lowerplaten Ill, an upper crosshead H and side rods l2 whereby this whole frame moves asan integral unit with rain 8 during loading or unloading of a. specimen; A sensitive frame has an intermediate crosshead l8 ad- Justably supported on threaded rods M which in turn are secured to a lower cro'sshead l5 while usual preloadingsprings l6, interposed between usual flexible stay plates H and a cylinder base i8, urge the sensitive frame upwardly to bring lower crosshead l5 into contact with a movable portion of hydraulic support 9 to preload the same. The ends of the stay plates ll are secured to cylinder E and to rods i l thereby to laterally guide and support the sensitive frame during its minute movements. As shown the hydraulic support 9 is seated against and forms the head of cylinder 7. The pumps broadly comprise hydraulic means for subjecting the loading apparatus to a pulsating load while the controllers-t and 5 are broadly pressure responsive means.

The main pumping unit 2 normally supplies fluid under a constant pressure to testing machine cylinder 1 beneath ram 8 through pipes i9, 29 and ii in which a normally open stop valve 22 is disposed. A pressure relief valve 23 is adapted upon occurrence of an abnormally high pressure in pump 2 to bypass fluid therefrom through a drain pipe 25 to an oil sump 2?. To determine the load upon the specimen, a pipe 22 normally connects the fiuid space of the hydraulic support with a pressure gauge as shown in said Emery patent, While my invention is herein applied to the foregoing Emery type of machine to show the applicability thereto, yet itrwill be understood that my invention is equally applicable to simple ram and cylinder types of testing machines wherein the load on a specimen is determined directly by the pressure in the main cylinder, this being illustrated herein by a piping system having suitable valves that may be opened or be described first for a simple ram and cylinder type of machine which is accomplished by closing a valve 28, Fig. 1. Main pump unit 2 is first started whereupon it supplies hydraulic pressure fluid through pipes I9, and 2! to the main loading cylinder 7 and also past a normally open valve to an inlet pipe 3|, thence through a ball check chamber 32 to passages 33 and 34 and past an adjustable needle valve 35 for controllin a by-pass passage 35'. This by-pass is connected to a minimum pressure outlet pipe which leads to the minimum pressure indicator and controller 5, Figs. 1 and 2. The controller 5, per se, is well known and does not form a part of the invention except as an element of my improved combination. The controller consists of pipe 36 connected, as shown in Fig. 2, to a Bourdon tube 31 whereby upon increase of pump pressure the Bourdon tube expands to cause its free end 38 to move a pivotally connected link 39 downwardly, thereby swinging an indicating pointer Ml to the right about a pivot M. The indicating pointer as shown in Fig. 1 moves over a dial 42 suitably graduated for pressure readings. Also during movement of pointer ll] a pivotally connected link 43, Fig. 2, moves an arm M about a stationary pivot 65. This pivot 65 also pivotally supports an adjustable frame A6 which carries both an air jet ll and a jet baflle 48. The means for carrying the bafile by frame 56 consists in pivotally supporting the bafiie as at 49 upon a vertically adjustable plate 50 which is suitably guided and supported upon the frame 46. A spring 5| biases battle 58 in a counterclockwise direction until a finger 52, depending from and forming a part of the baffle, engages a pin 53 which is carried by arm 44 and projects inwardly into the path of finger 52. Link M overlies the baffle and arm 52. To adjust the predetermined pressure limit which the controller is to maintain, the lower end of pivotal frame 66 may be swung laterally in either direction through a rack and pinion 5 3 operated by a suitable knob "5 located on the outside of the instrument. Lateral movement of frame i5 causes baiiie 68 to be initially positioned either closer to or further from jet il, thereby adjusting the pressure limit in a manner to be presently described. To adjust the sensitivity of the action of the baffle 48 in response to a given movement of arm 44, plate 50 may be adjusted vertically in either direction by a rack and pinion 56 operated from a knob 51. Vertical movement of plate 50 will carry pivot t9 therewith and accordingly move baiile 88 and finger 52. Stop pin 53, by reason of being carried by link M, will not be moved upon vertical adjustment of plate 50.

As the pressure of pump 2 increases, arm 44 and pin 53 are moved to the right about pivot thereby allowing spring 5| to move finger 52 and battle 48 in a counterclockwise direction about the normally stationary pivot 49. Such counterclockwise movement causes baffle 48 to move toward jet 4? to increasingly restrict air flow therefrom. During this baiile movement finger 52 will continuously bear against pin 53 so that the movement of arm 44 controls the extent of movement of the baflle. However, if the movement of arm 44 should be carried so far that the baliie actually engages the jet structure, then any further movement of arm 44 to the right will simply cause pin 53 to move away from finger 52 without any additional force being exerted on the baffle except the very slight force of spring 5|. The aforesaid increasing restriction of the Jet clauses an increase of air pressure in a pipe 51 in a manner that is well known in the air relay art. Pipe 51 is connected to a diaphragm chamber 58 whose diaphragm 58 is connected to a ball valve 59. Said increase in pressure moves ball valve 59 upwardly toward its valve seat, thereby to wholly or partially shut off flow of pressure air from a supply pipe 60 in which the pressure is maintained substantially constant by flowing from a suitable source through a pressure reducing valve 15. Normally, the supply of air would flow through passage 6| past valve 59 and into passages 62 and 63 and thence through a pipe 64 to an air operated servo-motor 65, Figs. 1 and 3, thereby causing a diaphragm 66, Fig. 3, to adjust or hold pumping unit 2 away from its neutral position through a wedge cam 61 and a follower shaft 68. However, when the supply of air past valve 58 is increasingly restricted as aforesaid by an increasing pressure in Bourdon tube 31 and in diaphragm chamber 58, then fluid in passages 62 and 63 as well as in servo-motor 65 will be vented through a clearance 18 at a faster rate than the rate at which source fluid flows from passage 6| past valve 59. This will result in a drop of pressure in the servo-motor so that a spring ll, Fig. 3, will move wedge 6! upwardly to allow a spring 12 to adjust pump 2 toward the right, thereby reducing the displacement thereof. When the pump pressure reaches a predetermined value, referred to herein as the predetermined minimum pressure, the Bourdon tube will have caused jet t1 and valve 59, Fig. 2, to be completely or sufllciently closed so that the pump is in its neutral position.

With the predetermined minimum pressure now established, pulsator pump 3, Fig. 1, can be operated. Various types of pulsators may be employed for producing pressure fluctuations along a sine curve although in order to take advantage of and obtain the complete sine curve action, it is necessary to use such a pulsator in a cooperating control system such as disclosed herein. For purposes of illustration the pulsator pump shown herein is specifically of the type described in the application of H. J. Ruch, Serial No. 171,765, filed October 29, 1937, now Patent No. 2,194,914. Hence it need be only briefly described as comprising a rotating cylinder 38 carrying a piston 81 which is reciprocated by a follower roller 82 running upon a circular trackway 83. This trackway can be adjusted so that its center coincides with the axis of rotation of cylinder about a shaft 84, thereby creating a neutral or non-pumping position of the pump, or its center may be variably shifted laterally to the right of shaft 84 to provide an eccentric relation to effect any desired degree of pulsator action. The eccentric adjustment of the trackway 83 is specifically accomplished either by downward movement of a cam wedge 85 acting on a follower 86 thereby moving the pump to the right away from its neutral position, or by a spring 81 adapted to move the pump to the left to decrease its eccentricity upon upward movement of cam 85 by a spring 99. The pulsator cylinder 80 is in constant open communication with the testing stant.

' Fig. 1, through valve 3t, pipe 3!, chamber 32 and a passage 92 from which fluid flows upwardly to raise a check valve 98 and thence flows through a valve chamber to a maximum pressure outlet pipe 95. The pipe d5 is connected to a Bourdon tube in controller d in the same manner that pipe as, Fig. 2, is connected with Bourdon tube 3'5. li'he controller d is identical in construction and operation as that shown in Fig. 2 and hence it will sumce to refer to Fig. 2 in describing the manner in which the maximum pressure is controlled.

Assuming that the pulsator is not-adjusted to produce a piston displacement which will give the desired maximum pressure, then the Bourdon tube in controller 8 will not be sufiiciently expanded to restrict flow from the air jet and accordingly the air pressure supplied through pipe 55' (corresponding to pipe 56 of Fig. 2) will communicate freely with a servo-motor pipe 55 (corresponding to pipe 66 of Fig. 2). This air pressure in pipe M will move a diaphragm 9? of a servo-motor 95 downwardly against the action of a spring 99, thereby to cause wedge 85 to move follower 86 to the right with consequent increase in eccentricity of track 83 and displacement of fluid from the pulsator. The eccentricity will continue to increase with an accompanying increase of maximum pressure during each pulsa-- tion, until the desired pressure is obtained.

.However. as the pressure builds up it is transmitted through valve 30, pipe 3|, chamber 32,

passage 92 and chamber 94 to pipe 95, thereby expanding the Bourdon tube in controller 4 so as to restrict flow offluid through the air jet supply pipe 60' with consequent restriction of air flow through pipe 84'. When the desired pressure is reached, the relation between the baflie and jet will be such as to hold a valve (conesponding to 59, Fig. 2) in a position whereby the servo-motor pressure in pipe 64' will be con- Such constant pressure will allow diaphragm 91 to remain in a fixed position and accordingly the pump. will be held in an adjusted position so that its eccentricity will be just sufficient to create the desired maximum pressure at the end of each stroke of piston 8|. The end of the stroke is considered to be at the radially inner end of pulsator cylinder 80. The receding or outward movement of pulsator piston 8| allows the loading pressure to drop down to the minimum established by pump 2. If for any reason, the maximum pressureshould fall below the desired predetermined value, the Bourdon tube in controller 4 will'contract thereby moving the battle away from its jet so as to allow the valve (corresponding to valve 59 of Fig. 2) to increase flow of fluid to pipe 64' thereby increasing the stroke of the pulsator piston. When the desired maximum pressure is reestablished, the

Bourdon tube will have expanded to reestablish the stabilized relation between the jet and ba-file. If for any reason the pulsator should produce a pressure overshooting the predetermined maximum, the Bourdon tube in controller 4 will have expanded so as to move the bafile nearer to the jet with consequent increased restriction of air flow therethrough resulting in the air relay (corresponding to 5B, 58' and 59) restricting flow of air from supply pipe 60' to servo-motor pipe 56'. As a result-the pressure acting on top of diaphragm 9'3, Fig. 4, will drop slightly whereupon spring 99 will move cam 85 upwardly to permit spring 81 to reduce the eccentricity of trackway 83 and the displacement of the pulsator piston with a consequent reduction in the maximum pressure to the desired value.

Maximum and minimum pressure control valve or filtering mechanism Inasmuch as the minimum pressure controller 5 and the maximum pressure controller d are each subjected to the pressure variations caused by the pumping units 2'2 and 3 as well as being commonly connected to the same source, such as the pumps, through the same inlet pipe 38, the control valve or pressure filter mechanism 8 is so arranged as to permit the controller 5 to be responsive only to minimum pressures and the controller 5 responsive to only maximum pressures. For example, minimum pressure fluid I cannot flow past check valve 32 to minimum pressure outlet pipe 3% but can only flow thereto through passages 33, 35 and past a restricted orilice, controlled by needle valve 3%, to passage 35'. Hence so long as the pressure in pipe 3| is below the predetermined minimum value, pump 2 will supply fiuid that flows past needle valve 35 to pipe 35 and to the Bourdon tube in controller 5. If for any reason the pressure in pipe 36 and the Bourdon tube 3? should increase slightly beyond the predetermined minimum, the main pump 2 will be moved to its neutral position as previously described. However, the systemhas a certain small amount of inherent leakage with the result that pressure in pipe 3! will gradually drop, thereby allowing the slightly excess pressure in pipe 36 to raise check valve 32' and establish the same pressure in pipe 35 as exists in pipe 3|. fallen slightly below the predetermined minimum, the Bourdon tube 31 will operate the jet and baffle 47 and 48 to adjust pump 2 so that it delivers-a small amount of liquid to the loading system. Consequently the pressurewill again flow through pipe 3| and past needle valve 35 to Bourdon tube 31, but this time the pressure in pipe 36 will not overshoot the predetermined minimum to the same extent as at the beginning of the pumping operation. The foregoing cycle will continue until the desired pressure is reached and stabilized. The restricted orifice controlled by needle valve 35 is. of course, a factor in stabilizing the operation because a momentary rise in pumping pressure above the desired minimum will be retarded in passing needle valve 35 and consequently the controller 5 has ahigh degree of stability incontrolling the operation of pump 2.

The maximum pressure 'is controlled-in the reverse manner to that for controlling the minimum pressure just described. Forinstance, the

'fluid from pulsator 3 flows freely through pipe 38,

chamber 32 and passage 92 to raise check valve 93 and thence flows through pipe to the Bourdon tube in controller 4. As the pressure of this fluid momentarily decreases during its series of pulsations, the pressure fluid which accumulates in pipe 95 will act downwardlyon top of ball check 83 to close communication between chamber 94 and passage 92. So long as the pressure in pipe 95 is below the predetermined maximum, the controller 5 will cause increas- When the pressure in pipes 30 and 38 has ing adjustment of the pulsator 3 until the desired pressure is reached, :but if the maximum pressure in pipe 95 should for any reason exceed the desired predetermined value, the Bourdon tube in controller 4 will adjust the jet-baflle which would give the necessary degree of displacement for maintaining maximum pressure. This drop of the maximum pressure will be relatively slight but will nevertheless be sufllcient to permit any excess pressure in pipe 95 to equalize with the maximum pressure in passage 92. This equalization is accompanied by fluid flowing rearwardly from pipe 95 through a passage I past an adjustable needle valve lUl and passage IM to the open pump passage 92. The reduced displacement of the pulsator and consequent reduction of pressure in pipe 95 will allow the jet-baflle mechanism of controller 4 to increase the displacement of the pulsator. This cycle of operations of first overshooting and then undershooting will very quickly become sta bilized at substantially the maximum pressure. The restricted orifice controlled by needle valve I0! is a factor in stabilizing the operation of the controller 3 because upon occurrence of a momentary drop of pressure in passage 92, reverse I flow from pipe 95 to passage I02 will be materially retarded by the restricted adjustment of needle valve IOI. Hence'before suificient fluid can flow reversely past valve lfll, the next wave of increasing pressure will occur so that, if necessary, ball check 93 will be lifted by the increasing pressure so as to increase the pressure in line 95 until the pressure therein is equal to the pressure in passage 92. This mode of operation is similar to that for the minimum pressure needle valve 35 which, of course, functions in the reverse manner as heretofore explained.

In the operations so far described it has been considered that the testing machine was operating as a simple ram and cylinder type wherein the cylinder pressure was taken as representative of the load on the specimen. This required opening of valve 30 and closing of valve 28' to discomiect the hydraulic support 9. However, if the independent weighing action of support 9 is to be employed, then valve 28 is opened and valve 30 closed. The pumps are still controlled in accordance with the maximum and minimum load pressures because these pressures are now transmitted from support 9 to valve 6. All other operations remain the same as before.

From the disclosure herein it is seen that two pumping mechanisms are provided one capable of producing only a predetermined minimum value and the other being capable of producing a pulsating maximum pressure together with a pressure controlling and responsive system that insures ease in adjustment and accuracy and sensitivity in operation. The pressure selector control mechanism functions in the nature of a filter for segregating the maximum and minimum pressures from each other. In the specific embodiment of the invention, each of the pumping mechanisms is adjusted by controllers which are respectively responsive only to maximum and minimum pressures, the adjustment of the pumps being such that the pressure pulsations are along a true sine curve regardless of what the maximum or minimum load value may be.

'controller 5, Fig. 1.

This true sine curve is accomplished by reason of each pump being so adjusted or operated that they respectively independently'produce only one pressure limit. In certain prior art pulsating and pumping arrangements, a relief valve is required in the system for cooperation with the pulsator in order to establish the maximum pressure limit, but this has the objectional feature that a true sine curve is not obtained because the relief valve functions to discharge pressure in excess of the desired maximum. This is equivalent to creating flat pressure lines across the top of the sine curve instead of a smooth continuous true sine such as is insured in the present combination. Furthermore, it is not necessary in my present combination, as is the case with the above prior art arrangement, to have a continuous rate of discharge from the main pumping unit 2 after its predetermined minimum value is established. -This is because pump 2 is either returned to its neutral position or the discharge is otherwise controlled so as to in effect disconnect the pump from the loading system except for such minor leakage as may occasionally occur.

For example. if desired, pump 2 may be of the continuously operating constant displacement type. In this case, as diagrammatically shown in Fig. 5, the predetermined minimum pressure is established by automatically controlling a valve 23' by a servo-motor which corresponds in every respect to servo-motor 55. The valve is located in the drain pipe 25 so that when the predetermined pressure is reached, valve 23' is opened to bypass fluid from the pump through drain pipe 26 and when the pressure falls below said predetermined value, valve 23 moves toward its closed position to restrict or close pipe 26. A further arrangement as diagrammatically shown in Fig. 6, equivalent to stopping pump 2 or disconnecting the same from the loading system, is to insert in pipe 20 a stop valve 18 operated by a servo-.motor TI. This servo-motor is connected to pipe 64 whereby when the predetermined minimum pressure is reached the valve 16 closes pipe 20 and by-passes fluid from the pump to a drain 26'. When the pressure drops below said predetermined minimum, the stop valve I6 opens to connect pipe ill with pipe 20 and at the same time close the discharge through drain 26'. If desired, the valve 16 could be manually operated by adjusting a usual handwheel 16a and threaded valve stem 16b, the operator observing the pressure indication of pointer 40 of After the minimum pressure is obtained, adjustment of any of the stop valves 16 or valve 23' or the adjustment of the variable displacement pump 2 by the wedge i1 is merely for the purpose of supplying make-up fluid to take care of the leakage if such should occur. Broadly the pump and valve adjusting mechanisms comprise means for controlling or varying the rate of fluid flow to the load producing apparatus.

It will of course be understood that various changes in details of construction and arrangemeans respectively controlling said pumps in acbly controlling the maximum and minimum limits of the hydraulic pressure produced by said vtinuously and uninterruptedly through a complete cycle, and means for controlling one of said pumps in accordance with a predetermined maximum pressure and for controlling the other of said pumps in accordance with a .predetermined minimum pressure while maintaining said continuous and uninterrupted curve through all variations of load 3. In combination, a hydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load between predetermined maximum and minimum pressure limits including a pump for producing a substantially constant pressure and a pulsator pump for producing a pulsating pressure along a true sine curve extending continuously and uninterruptedly through a complete cycle, pressure responsive means for controlling one of said pumps so as to produce said predetermined'minimum pressure while the pressure i maintained along said continuous and uninterrupted curve, and pressure responsive means for controlling the other of said pumps to produce said predetermined maximum pressure while the pressure is maintained along said continuous and uninterrupted curve.

4. In combination, ahydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load including a pump for producing a substantially constant pressure and a pulsating pump for producing a pulsating pressure along a true sine curve extending continuously and uninterruptedly through acomplete cycle, a plurality of pressure responsive 4;

cordance with predetermined maximum and minimum pressure limits while maintaining the pressure along said continuous and uninterrupted curve, and means operative to transmit high pressure to one of said responsivemeans and to transmit low pressure to the other re,-v sponsive means. I

5. In combination, a hydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load between predetermined maximum and minimum limits, and meansfor controlling said hydraulic means so as to produce said pressure limits including a pressure filtering mechanism operative to transmit maximum and minimum load pressures separately from each other. 6. In combination, a hydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load between predetermined maximum and minimum limits, and means for controlling said hydraulic means so as to produce said pressure limits including a pressure filtering means having maximum and minimum pressure outlets supplied from a common pressure fluid source which is subjected to all pressure fluctuations between said pressure limits and means for conducting only the maximum and minimum limits of pressures to said outlets.

7. In combination, a hydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load between predetermined maximum and minimum limits, and

means for controlling said hydraulic means so as to produce said pressure limits including a pressure filtering means having maximum and minimum pressure outlets supplied from a common sourceof pressure fluid, acheck valve and restricted by-pass around the same associated with one of said outlets to allow free flow of fluid theretobut restricted flow therefrom and another check valve andrestrictedby-pass around the same associated with the other of said outlets to allow restricted flow of fluid thereto but free-flow therefrom, whereby the maximum and minimum pressures of said common source are adapted to be segregated.

8.. The combination set forth in claim 7 further characterized in that said by-passes are provided with means for varying the rate of bypass action around the check valves.

9. In combination, a hydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load, and means for controlling said hydraulic means so as to maintain said pulsating load between predetermined maximum and minimum limits, said control means including means for varying the rate of fluid flow from the hydraulic means to said load producingapparatus and an air jet and bafile for controlling said rate of flow means.

10. The combination set forth in claim 3 further characterized in that each of said .pres-. sure responsive means includes a hydraulically operated Bourdon tube connected to the hydraulic means and an air jet and battle controlled by said Bourdon tube, and means controlled by said air jets and baflies ;for controlling said hydraulically pumping means.

11. In combination, a hydraulic load producing apparatus, hydraulic means for subjecting said apparatus to a pulsating load between predetermined maximum and minimum pressure limits including a pump for producing a subfor producing a pulsating pressure, pressure responsive means for controlling one of said pumps so as'to produce said predetermined minimum pressure. and pressure responsive means for controlling the other of said pumps to, produce said predetermined maximum pressure; each of said pressure responsive means comprising a hydraulically operated Bourdon tube connected to the hydraulic means, an air control mechanism having an air jet and baflie controlled by said Bourdon tube, and means for adjusting the re lation between said jet and baille for establishing a predetermined pressure limit.

12. The combination set forth in claim 2 further characterized in that said pump for producing a constant fluid pressure is of the type having variable displacement from zero to maximum and said control means includes mechanism for reducing the displacement of said pump as one of said predetermined pressure lim- 

